1. Field of the Invention
The present invention relates to filtering devices, membranes, and technology for use in separating fluids and gases.
2. Description of the Art
Separation of miscible compounds has traditionally relied on methods such as distillation, liquid-liquid extraction, crystallization/precipitation, gas stripping, adsorption, and membrane permeation. Each method has its range of applications and certain advantages over other methods. Differences depend on such factors as the nature of the mixture to be separated, the concentration of the components, the end uses of the components, the degree of separation/purification desired, economic factors, and so on.
Common applications of membrane technology intended for the separation of miscible liquid compounds are solvent dehydration and VOC removal from wastewaters, i.e. removal of the minor component of a mixture via selective permeation. Pervaporation is commonly used to remove water from the ethanol-water azeotropic mixture to yield fuel-grade ethanol.
Ethanol recovery and purification from a fermentation broth is a common operation, and increasingly so with the growth of fuel ethanol derived from grain or biomass saccharification and fermentation. Fermentation is typically run in batch-mode, and the resulting fermentation broth is fed gradually into a distillation column, or a series of columns. Ethanol is removed from this broth, but to do so a large portion of the balance (mainly water) must also be vaporized. As the overhead product approaches the ethanol-water azeotrope of about 96 wt % ethanol, the reflux ratio requirement rises dramatically, requiring larger and larger amounts of liquid to be vaporized and condensed. Fuel ethanol plants are typically operated such that distillation yields a stream that is somewhat below the azeotropic ethanol concentration; dehydration is accomplished by pervaporation or adsorption. Thus there is an optimum distillation operating concentration that balances the costs of distillation with the costs of dehydration. This is a very energy-intensive process, which is an undesirable quality when the goal is fuel production. Distillation, therefore, is not the ideal separation method for fuel ethanol.
Liquid-liquid extraction and membrane permeation are the two most commonly considered alternatives to distillation. Both are best utilized in removal of a minor component from a mixture. These methods are both less energy-intensive than distillation, and they also lend themselves to continuous or semi-continuous operation of the fermenter, with demonstrated increases in productivity and yield. They are not without their shortcomings, however.
Liquid-liquid extraction, for example, is generally a complicated and inefficient multi-step process: the feed is contacted with the solvent (usually with vigorous agitation to promote increased mass transfer via high surface area due to small droplet sizes, and convective as well as diffusive mass transfer), the solvent and feed phases are disengaged (usually by settling), and the product is removed from the solvent phase by distillation, gas stripping, flashing, a temperature shift to promote a phase separation, or other means. In addition, liquid-liquid solvent extraction is often further complicated by the formation of emulsions, which make phase-separation difficult and also lead to enhanced solvent losses to the feed mixture.
Membrane permeation methods, such as pervaporation, are alternatives which have been studied for ethanol recovery from dilute aqueous solutions. Pervaporation involves contacting the feed with a large amount of membrane surface area, permeating the product through the membrane into a vacuum or sweep gas, condensing the permeate vapor, and supplying vacuum for initial startup and removal of noncondensibles during operation. The process is appealing due to its simplicity, but the membranes themselves are expensive to manufacture and suffer from a short lifetime, often requiring replacement every 2-4 years.
A basic technical problem that neither liquid-liquid extraction nor conventional pervaporation has been able to overcome is producing ethanol above 95 wt % purity from a typical fermentation feed of 5-12% ethanol in one step. Of course, it is possible to add additional steps in order to achieve the desired purity level, but to do so requires additional equipment and therefore additional cost.
For membrane permeation, membrane modules have generally been manufactured using one of the following basic configurations: plate-and-frame, tubular, capillary, hollow fiber, and spiral wound. See R. Baker, Membrane Technology and Applications, 2nd ed, 2004. Each configuration has its own advantages and disadvantages.
Plate-and-frame and tubular modules tend to be the most expensive, with spiral-wound, capillary and hollow fiber modules being less expensive. For liquid separations, hollow fiber systems are relatively more prone to fouling and concentration polarization compared to all other designs. When the permeate is carried in the fiber lumen, the permeate-side pressure drop in hollow fiber systems is much higher than in spiral-wound modules; this reduces the driving force for separation, and thus negatively affects flux. Hollow fiber and capillary systems are limited to certain specific types of membrane materials (due to the manufacturing method, which requires high-speed automated spinning and fiber handling equipment), whereas spiral-wound, tubular and plate-and-frame designs are more versatile.
Spiral-wound configurations are notable because they provide a higher ratio of membrane surface area to volume of the module, compared to flat-plate and tubular modules. Hollow fiber modules provide even higher ratios, but this is often offset by the lower fluxes achieved in them, which then require additional surface area, negating the advantage over spiral-wound systems.
Various membrane designs have been attempted, but for each there appear to be shortcomings.
For example, U.S. Pat. Nos. 4,750,918, 4,789,468, and 5,637,224, all to Sirkar et. al., relate to the use of hollow fiber/liquid membrane pervaporation for the removal of volatile organic compounds from aqueous solutions. They describe a combination process of solvent extraction to extract volatile organic compounds (VOCs) from wastewater coupled with pervaporation to remove the VOCs from the extracting solvent. The module is the hollow-fiber type. The '224 patent in particular discloses a device whereby the feed (wastewater solution containing VOCs) flows through the center of a porous hollow fiber. Surrounding the hollow fiber is the extraction solvent. Either the feed, or the solvent, preferentially wets the pores of the hollow fiber. VOCs are extracted through the pores and into the solvent, and the resulting retentate (purer water) exits the hollow fiber at the other end of the module. A second hollow fiber, whose wall is a nonporous pervaporation membrane, runs through the module parallel to the first hollow fiber. The inside of this second hollow fiber is connected to a vacuum source. The VOCs in the extraction solvent external to this second hollow fiber permeate through its walls and into its center, and exit as vapor to the vacuum source. This device has several shortcomings, however. For example, it has limited versatility since it appears to be limited for use in wastewater remediation, not recovery of bio-based chemicals. Moreover, the device would not be suitable for use with fermentation broths, as it would be susceptible to fouling and clogging, as the feed mixture must flow through hollow fibers of extremely small diameter (100-300 microns in the example). Finally, the device is prone to destabilization of the interface (i.e., flow of one phase into the other through the pores), and requires operation such that a pressure difference across the porous membrane must be maintained at a sufficiently low value to prevent destabilization.
U.S. Pat. No. 5,580,452 to Lin (1996), “Moving Liquid Membrane Modules” describes an extraction system wherein a component of a feed solution is extracted through a porous membrane into a solvent, then removed from the solvent by back-extraction through a second porous membrane into a second fluid. Additionally, the solvent contained between the two membranes is flowing in order to increase mass transfer. A spiral-wound module is described, as is a tubular module form where the membranes are concentric tubes. The feed flows through the center of the inner tube, the solvent flows between the two tubes, and the strip fluid is outside the second tube. A hollow-fiber module is also detailed. This configuration results in the extracted components being removed by back-extraction into another fluid, via a porous membrane, which in turns leads to a further separation step in order to isolate the permeated components from the strip solution.
The device described by P. Christen, M. Minier et. al. in “Ethanol Extraction by Supported Liquid Membrane During Fermentation”, Biotechnol. Bioeng., 36, pp 116-123 (1990), is a supported liquid membrane in a flat-plate configuration, which is used to remove ethanol from a fermentation broth. Two modes of operation are described. The first method is perstraction whereby water is circulated on the permeate side of the membrane, thus back-extracting the ethanol from the supported liquid membrane. In this mode, however, the supported liquid membrane has proven to be unstable, with gradual loss of solvent from the pores of the supporting membrane. The second mode employs pervaporation, whereby air is circulated on the permeate side of the membrane, thus evaporating the ethanol from the supported liquid membrane. The supported pervaporation liquid membrane is stable, but fine control of the pressure difference across the membrane is necessary in order to prevent ejection of the solvent from the pores, and such control is quite difficult to achieve. Moreover, there is no method to replenish extraction solvent lost from the pores of the supported liquid membrane, and in perstraction mode the ethanol must still be separated from the water used in order to strip it from the extraction solvent. The permeated components thus are not obtained directly. Additionally, in pervaporation mode, recovery of the ethanol is difficult because the ethanol vapor is in a relatively low concentration in the sweep gas. Finally, the condenser used must have a relatively large surface area and operate colder, compared to the present invention which operates under vacuum.
The device described by T. Itoh et. al. in “Permselectivity of Liquid-Polymer Hybrid Membrane Composed of Carbon Tetrachloride and 2-Hydroxyethyl Acrylate-Acrylonitrile Graft Copolymer for Ethanol-Water Mixture”, Polym. J., 15, pp 827-834 (1983) details the extraction of ethanol from an ethanol-water mixture into a layer of solvent (carbon tetrachloride), and the further permeation of the ethanol from the solvent through a nonporous membrane by pervaporation. The solvent in this device is not contained in the pores of a supporting membrane and the measured flux was extremely low. Hence, this type of configuration is not suitable for commercial applications.
Given the shortcomings of each of the preceding devices, what is needed is a simple, efficient, and economical system with low energy requirements for separating miscible compounds.